High-strength hot-rolled steel sheet and method for producing same

ABSTRACT

The present invention provides a high-strength hot-rolled steel sheet having both excellent strength and excellent workability (particularly, bending workability), and a method of producing the same. 
     The steel sheet of the present invention has a certain composition as well as microstructures such that an area ratio of ferrite phase is 95% or more, an average grain size of the ferrite phase is 8 μm or less, and carbides in grains of the ferrite phase have an average particle size of less than 10 nm. The steel sheet of the present invention also has a tensile strength of 980 MPa or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2013/000257, filedJan. 21, 2013, which claims priority to Japanese Patent Application No.2012-013592, filed Jan. 26, 2012, the disclosures of each of theseapplications being incorporated herein by reference in their entiretiesfor all purposes.

FIELD OF THE INVENTION

The present invention relates to a high-strength hot-rolled steel sheetthat has both high strength, or a tensile strength (TS) of 980 MPa ormore, and excellent workability (particularly, bending workability), andis usefully applied in automobile members, and a method for producingthe same.

BACKGROUND OF THE INVENTION

In recent years, to reduce CO₂ emission from the viewpoint of globalenvironment protection, there is an increasing demand in the entireautomobile industry for improved fuel efficiency of automobiles. Toimprove fuel efficiency of automobiles, it is most effective to reducethe weight of automobiles by reducing the thickness of members used inthe automobiles. Accordingly, high-strength hot-rolled steel sheets havebeen increasingly used as materials for automobile components. On theother hand, since most of automobile members made by steel sheets areformed by press forming or the like, the steel sheets for automobilecomponents are required to have high strength as well as excellentbendability (bending workability).

In general, however, ductility and workability are reduced as steelmaterials have higher strength. This may cause difficulties in forming asteel sheet with enhanced strength of an increased tensile strength of980 MPa or more into the desired shape of components. For example, inthe case where a steel sheet having a tensile strength of 980 MPa ormore is subjected to press forming, it is difficult to form the steelsheet into the shape of components due to significant cracking, necking,and so on occurring at bending-processed portions.

For the reasons noted above, it has been desired to develop ahigh-strength steel sheet that exhibits both desired strength andexcellent bending workability when applied to automobile components orthe like, and various techniques have been proposed to date.

For example, JP 2007-262429 A (PTL 1) proposes a technique where a steelsheet is arranged to have a chemical composition containing, in mass %,C, 0.05% to 0.20% and Nb: 0.1% to 1.0%, and that the content of solute Cis 0.03% or less. According to the technique proposed by PTL 1, it issaid that by limiting the content of solute C by a chemical systemcontaining Nb and C, such an abrasion-resistant steel sheet is obtainedthat has microstructures in which the matrix is ferrite phase which issoft in nature, and NbC is dispersed in the matrix as a hard secondaryphase, and that have excellent bending workability.

In addition, JP 2008-189978 A (PTL 2) proposes a technique whereby asteel sheet is arranged to have a chemical composition containing, inmass %, C, 0.02% to 0.2%, Si: 0.01% to 1.0%, Mn: 0.1% to 2.0%, P: 0.2%or less, sol. Al: 0.001% to 0.5%, Ti: 0.1% or less, Nb: 0.1% or less, V:0.5% or less, Mo: 0.5% or less, and Ti+Nb: 0.1% or less, and havemicrostructures with ferrite as the main phase, where an average grainsize of ferrite within a region from a surface of the steel sheet to a ¼depth of the thickness of the steel sheet and an increasing rate of theaverage grain size at 700° C. are defined. It is stated in the techniqueproposed by PTL 2 that a steel sheet having excellent workability isobtained.

PATENT LITERATURE

-   PTL 1: JP 2007-262429 A-   PTL 2: JP 2008-189978 A

SUMMARY OF THE INVENTION

However, the technique proposed by PTL 1 involves substantiallyenhancing the strength of a steel sheet by dispersing NbC, and it isdifficult to obtain a steel sheet having a tensile strength of 980 MPaor more by using this technique utilizing NbC. This is because while thedegree of precipitation strengthening achieved by dispersingprecipitates increases with increasing carbide volume fraction, it isnot possible to increase the carbide volume fraction due to a smallsolubility product in steel and a large atomic density of NbC.

In addition, in the technique proposed by PTL 2, Ti and V are added tosteel as precipitation-strengthening elements, but Ti and V for formingcarbides are contained in the steel in a small amount, or added to thesteel in an inappropriate manner, in which case, again, the tensilestrength of the steel sheet does not reach 980 MPa.

As described above, in the conventional techniques, it was difficult toobtain a high-strength steel sheet having a tensile strength of 980 MPaor more. Moreover, it was not possible to impart excellent bendingworkability to the steel sheet, while retaining such high strength ofthe steel sheet.

The present invention has been made in view of these situations, and anobject of the present invention is to provide a high-strength hot-rolledsteel sheet that has a tensile strength of 980 MPa or more, and stillexhibits excellent bending workability.

To solve the aforementioned problems, the inventors of the presentinvention have focused on a technique for enhancing the strength of aferrite single-phase steel sheet having good workability by achievingfine particle distribution of carbides therein, and made intensivestudies on various factors that have an effect on enhancement of thestrength of the steel sheet and workability, particularly bendingworkability, of the steel sheet.

Then, the inventors have found that for the purpose of obtaining a hard,ferrite single-phase steel sheet from the ferrite phase that wouldnormally be soft in nature, it is extremely helpful to allow fineparticle distribution of carbides in the ferrite phase, and, as a resultof search for elements that allow precipitation of a large amount offine carbides, titanium (Ti) was identified as the most suitable elementfor this purpose.

However, since it was difficult to provide a ferrite singe-phase,hot-rolled steel sheet with a tensile strength as high as 980 MPa ormore by using Ti carbides alone, the inventors of the present inventionhave searched for ways to reinforce dispersion and precipitationstrengthening by means of Ti carbides.

As a result, the inventors have conceived an idea of adding vanadium (V)as reinforcing means. It is hard for V to precipitate when added alonedue to its high solubility in steel, whereas it becomes easier for V toprecipitate when coupled with Ti carbides. As a result, when Ti and Vare added in combination to the steel material of the hot-rolled steelsheet in an appropriate amount, the strength of the steel sheet isdramatically increased as compared to the case where Ti or V is addedalone, whereby a hot-rolled steel sheet having a tensile strength of 980MPa or more is obtained.

It was also found to be important to contain Ti in an amount equal to ormore than the content of V since precipitation of V is facilitated whencoupled with Ti carbides.

In addition, the inventors of the present invention have searched forways to impart excellent bending workability to a high-strengthhot-rolled steel sheet having a tensile strength of 980 MPa or more, towhich Ti and V have been added in combination as described above, whilemaintaining the strength of the steel sheet. As a result, to givebending workability, it was found advantageous to improve the surfaceappearance quality of the steel sheet, and furthermore, reduce soluteelements that would deteriorate the workability of the steel sheet andreduce inclusions as much as possible that would serve as the origins ofvoids. As a result of further investigations, it was revealed that ahot-rolled steel sheet that has a tensile strength of 980 MPa or moreand exhibits excellent bending workability may be obtained by arrangingthe steel sheet to have a component composition with optimized contentsof C, Mn, Ti and V, while reducing Si content as much as possible.

The present invention has been completed based on the aforementioneddiscoveries and the primary features thereof are as follows.

[1] A high-strength hot-rolled steel sheet having excellent bendability,the steel sheet comprising a chemical composition containing, in mass %,

C: 0.06% or more and 0.1% or less,

Si: 0.09% or less,

Mn: 0.7% or more and 1.3% or less,

P: 0.03% or less,

S: 0.01% or less,

Al: 0.1% or less,

N: 0.01% or less,

Ti: 0.14% or more and 0.20% or less,

V: 0.07% or more and 0.14% or less, and

the balance being Fe and incidental impurities,

wherein the steel sheet has microstructures such that an area ratio offerrite phase is 95% or more, an average grain size of the ferrite phaseis 8 μm or less, and carbides in grains of the ferrite phase have anaverage particle size of less than 10 nm, and

wherein the steel sheet has a tensile strength of 980 MPa or more.

[2] The high-strength hot-rolled steel sheet having excellentbendability according to [1] above, wherein the chemical compositionfurther contains, in mass %, Nb: 0.01% or more and 0.05% or less.

[3] The high-strength hot-rolled steel sheet having excellentbendability according to [1] or [2] above, wherein the chemicalcomposition further contains at least one of Mo, W, Zr and Hf, thecontent of each element being controlled so that Mo: 0.05% or less, W:0.05% or less, Zr: 0.05% or less, and Hf: 0.05% or less.

[4] The high-strength hot-rolled steel sheet having excellentbendability according to any one of [1] to [3] above, wherein thechemical composition further contains, in mass %, at least one of O(oxygen), Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Ti, Zn, Cd, Hg, Ag, Au,Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re, Ta, Be, Sr, REM, B, Ni, Cr, Sb, Cu,Sn, Mg, and Ca, in a total amount of 0.2% or less.

[5] The high-strength hot-rolled steel sheet having excellentbendability according to any one of [1] to [4] above, further comprisinga plating layer on a surface of the steel sheet.

[6] The high-strength hot-rolled steel sheet having excellentbendability according to [5] above, wherein the plating layer is agalvanized layer.

[7] The high-strength hot-rolled steel sheet having excellentbendability according to [5] above, wherein the plating layer is agalvannealed layer.

[8] A method of producing a high-strength hot-rolled steel sheet havingexcellent bendability, comprising: heating a steel material; subjectingthe steel material to hot rolling including rough rolling and finishrolling; and after completion of the finish rolling, cooling and coilingthus rolled steel material to gain a hot-rolled steel sheet, wherein

the steel material has a chemical composition containing, in mass %,

C: 0.06% or more and 0.1% or less,

Si: 0.09% or less,

Mn: 0.7% or more and 1.3% or less,

P: 0.03% or less,

S: 0.01% or less,

Al: 0.1% or less,

N: 0.01% or less,

Ti: 0.14% or more and 0.20% or less,

V: 0.07% or more and 0.14% or less, and

the balance being Fe and incidental impurities, and

wherein the steel material is heated at a temperature of 1100° C. orhigher and 1350° C. or lower, the finish rolling is operated at a finishrolling temperature of 850° C. or higher, the cooling is initiatedwithin 3 seconds after completion of the finish rolling, the cooling isoperated at an average cooling rate of 20° C./s or higher, and thecoiling is operated at a coiling temperature of 550° C. or higher and700° C. or lower.

[9] The method of producing a high-strength hot-rolled steel sheethaving excellent bendability according to [8] above, wherein thechemical composition further contains, in mass %, Nb: 0.01% or more and0.05% or less.

[10] The method of producing a high-strength hot-rolled steel sheethaving excellent bendability according to [8] or [9] above, wherein thechemical composition further contains at least one of Mo, W, Zr and Hf,the content of each element being controlled so that Mo: 0.05% or less,W: 0.05% or less, Zr: 0.05% or less, and Hf: 0.05% or less.

[11] The method of producing a high-strength hot-rolled steel sheethaving excellent bendability according to any one of [8] to [10] above,wherein the chemical composition further contains, in mass %, at leastone of 0 (oxygen), Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Ti, Zn, Cd, Hg,Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re, Ta, Be, Sr, REM, B, Ni, Cr,Sb, Cu, Sn, Mg, and Ca, in a total amount of 0.2% or less.

[12] The method of producing a high-strength hot-rolled steel sheethaving excellent bendability according to any one of [8] to [11] above,comprising forming a plating layer on a surface of the hot-rolled steelsheet.

[13] The method of producing a high-strength hot-rolled steel sheethaving excellent bendability according to [12] above, wherein theplating layer is a galvanized layer.

[14] The method of producing a high-strength hot-rolled steel sheethaving excellent bendability according to [12] above, wherein theplating layer is a galvannealed layer.

According to the present invention, it is possible to obtain ahigh-strength hot-rolled steel sheet having a tensile strength of 980MPa or more and excellent bending workability that is suitablyapplicable to automobile structural members or the like, that is highlyadvantageous in, for example, being capable of reducing the weight ofautomobile members, forming automobile members or the like, and thatenables the even wider application of high-strength hot-rolled steelsheets, thereby causing a significantly advantageous effect inindustrial terms.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention will be described in detail below with referenceto exemplary embodiments.

(High-Strength Hot-Rolled Steel Sheet)

Firstly, description will be made of the selection of the preferredmicrostructures and carbides of the steel sheet of the presentinvention. The hot-rolled steel sheet of the present inventionpreferably has microstructures such that an area ratio of ferrite phaseis 95% or more, an average grain size of the ferrite phase is 8 μm orless, and carbides in grains of the ferrite phase have an averageparticle size of less than 10 nm.

Area Ratio of Ferrite Phase: 95% or More

The metal structure of the matrix of the hot-rolled steel sheet ispreferably ferrite single-phase structure having excellent workability.When a secondary phase, such as bainite phase, martensite phase,cementite or pearlite, is incorporated into the microstructures of thesteel sheet, voids are generated at interfaces between the ferrite phaseand the secondary phase having different hardness from each other, whichdeteriorates the bending workability of the steel sheet.

In addition, the present invention allows carbides, such as Ti and/or Vcarbides, to precipitate in the steel sheet in order to ensure thedesired strength of the steel sheet. Most of these carbides are suchcarbides that undergo austenite to ferrite transformation and interphaseprecipitation at the same time during a cooling step after completion ofthe finish rolling in the process of producing a hot-rolled steel sheet.Thus, it is beneficial to facilitate ferrite transformation to obtainmore carbides for the desired strength (tensile strength: 980 MPa ormore) of the steel sheet; if the area ratio of the ferrite phase isbelow 95%, it is difficult to ensure a tensile strength of 980 MPa ormore.

For the reasons given above, it is preferable in the present inventionthat the metal structure of the hot-rolled steel sheet is ferritesingle-phase structure. However, if the metal structure is not exactferrite single phase, it is still possible to obtain the desiredstrength (tensile strength: 980 MPa or more) as long as the ferrite arearatio is 95% or more. Therefore, the area ratio of the ferrite phase isto be 95% or more, preferably 98% or more.

In addition, in the hot-rolled steel sheet of the present invention,typical phases other than the ferrite phase that may be contained in thesteel sheet include cementite, pearlite, bainite, martensite, and so on.If such phases are present in the steel sheet in a large amount, theproperties (such as bending workability) of the steel sheet deteriorate.It is thus preferable to reduce such phases as much as possible,although a total area ratio of these phases is acceptable up to 5%, andis preferably 2% or less, relative to the entire metal structure of thesteel sheet.

Average Grain Size of Ferrite Phase: 8 μm or Less

A ferrite average grain size exceeding 8 μm more likely results inmixed-grain-size microstructures. Then, in such mixed-grain-sizemicrostructures, coarse ferrite grains are more susceptible to stressconcentration during bending working, which leads to a significantreduction in the bending workability of the steel sheet. Accordingly,the upper limit of the average grain size of the ferrite phase is to be8 μm. The average grain size of the ferrite phase is preferably 6 μm orless, more preferably 4.5 μm or less.

Carbides in Ferrite Grains

From the viewpoint of ensuring strength, the hot-rolled steel sheet ofthe present invention preferably allows fine precipitation of carbidesin the grains of the ferrite phase. In the present invention, thecarbides to be finely precipitated in the grains of the ferrite phasemay include Ti carbides, V carbides, composite carbides of Ti and V andcarbides further containing Nb, W, Mo, Hf and Zr. Most of these carbidesare such carbides that undergo austenite to ferrite transformation andinterphase precipitation at the same time during a cooling step aftercompletion of the finish rolling in the process of producing ahot-rolled steel sheet.

Average Particle Size of Carbides in the Ferrite Grains: Less than 10 nm

In the hot-rolled steel sheet of embodiments of the present invention,the above-described fine dispersion of carbides, mainly of compositecarbides of Ti and V, is used to enhance the strength of the steelsheet, where finer carbides provide more particles interfering withdislocation movement, and hence result in a higher degree of enhancementof the strength achieved by the dispersion of the carbides. Accordingly,in the present invention, for the purpose of providing the hot-rolledsteel sheet with the desired tensile strength (980 MPa), the averageparticle size of the carbides to be dispersed in the ferrite grains ispreferably less than 10 nm, more preferably less than 7 nm, and morepreferably 5 nm or less.

Next, description will be made on reasons for selecting the preferredchemical composition of the hot-rolled steel sheet of the presentinvention. As used herein, “%” in the following chemical compositionsmeans “mass %,” unless otherwise specified.

0.06%≦C≦0.1%

Carbon (C) is bonded to Ti, V or further to Nb to form carbides, whichpresent with fine particle distribution in the steel sheet. That is, Cis an element that forms fine carbides to significantly strengthen theferrite phase and is essential for enhancing the strength of ahot-rolled steel sheet. To obtain a high-strength steel sheet having atensile strength of 980 MPa or more, C content in steel is preferably atleast 0.06% or more. On the other hand, C content in steel exceeding0.1% causes precipitation of a large amount of cementite, whichdeteriorates the bending workability of the steel sheet. This is becausemicro-voids are generated more easily at interfaces between cementiteand the matrix (ferrite), and these micro-voids constitute a factor incausing cracks in the steel sheet at those portions subjected to bendingworking. Accordingly, the C content is to be 0.06% or more and 0.1% orless, preferably 0.07% or more and 0.09% or less.

Si≦0.09%

Silicon (Si) has been intentionally contained in conventionalhigh-strength steel sheets as an element that effectively improves thestrength of the steel sheets without deteriorating the ductility(elongation). However, Si tends to be concentrated on surfaces of thesteel sheets and form fayalite (Fe₂SiO₄) thereon. Since the fayalite isformed in wedge shape on a surface of the steel sheet, it would serve asthe origin of cracking when the steel sheet is being subjected tobending working. Accordingly, it is desirable in the present inventionto reduce Si content in steel as much as possible; however, up to 0.09%is acceptable and the upper limit of the Si content is to be 0.09%. TheSi content is preferably 0.06% or less. The Si content may be reduced toimpurity level.

0.7%≦Mn≦1.3%

Manganese (Mn) is an element that serves to refine carbides to beprecipitated in grains of the ferrite phase of a hot-rolled steel sheet,and thus effectively enhances the strength of the steel sheet. Asdescribed earlier, in the present invention, most of these carbidesprecipitated in the grains of the ferrite phase will undergo austeniteto ferrite transformation and interphase precipitation at the same timeduring a cooling step after completion of the finish rolling in theprocess of producing a hot-rolled steel sheet. Thus, if thetransformation occurs at a high temperature, carbides would beprecipitated at a high temperature range and experience coarseningduring the cooling step arriving at coiling.

To address these problems, since Mn has an effect of lowering thetemperature at which austenite to ferrite transformation occurs insteel, the transformation temperature may be lowered to a coilingtemperature range described later by containing a predetermined amountof Mn, and carbides may be precipitated concurrently with coiling of thesteel sheet. Then, such carbides precipitated concurrently with thecoiling without being exposed at a high temperature range for a longperiod of time would be kept in fine grain condition. To obtain ahot-rolled steel sheet having a tensile strength of 980 MPa or more byrefining carbides, Mn content in steel is preferably at least 0.7% ormore. On the other hand, Mn content in steel exceeding 1.3% leads to asignificant deterioration in the workability of the steel sheet due tosolute Mn, which makes it impossible to obtain the desired bendingworkability. Accordingly, the Mn content is to be 0.7% or more and 1.3%or less, preferably 0.8% or more and 1.2% or less.

P≦0.03%

Phosphorus (P) is a harmful element that serves as the origin ofintergranular cracking during working when segregated at grainboundaries and thereby deteriorates the bending workability of the steelsheet. It is thus preferable to reduce P content in steel as much aspossible. Accordingly, to avoid this problem, the P content ispreferably 0.03% or less, more preferably 0.02% or less in the presentinvention. The P content may be reduced to impurity level.

S≦0.01%

Sulfur (S) is present in steel as inclusions, such as MnS. Since suchinclusions are hard in nature, interfaces between the matrix and theinclusions serve as the origins of voids when the steel sheet is beingsubjected to bending working, which results in a deterioration in thebending workability of the steel sheet. Accordingly, it is preferable inthe present invention to reduce S content as much as possible. The Scontent is to be 0.01% or less, preferably 0.008% or less. The S contentmay be zero, in which case there is no problem.

Al≦0.1%

Aluminum (Al) is an element that functions as a deoxidizer. To obtainthis effect, Al is desirably contained in steel in an amount of 0.02% ormore. However, if Al content exceeds 0.1%, the adverse effect on thebending workability caused by inclusions, such as alumina, appears.Accordingly, the Al content is to be 0.1% or less, preferably 0.08% orless.

N≦0.01%

Nitrogen (N) is an element that is bonded to Ti, which is acarbide-forming element, to form coarse Ti nitrides at the steelmakingstage and inhibits formation of fine carbides, which results in asignificant deterioration in the strength of the steel sheet. Moreover,when the steel sheet is being subjected to bending working, voids aregenerated more easily at interfaces between the matrix and coarse Tinitrides, which adversely affects the bending workability of the steelsheet. Accordingly, it is preferable to reduce N content as much aspossible. The N content is to be 0.01% or less, preferably 0.008% orless. The N content may be zero, in which case there is no problem.

0.14%≦Ti≦0.20%

Titanium (Ti) is an element that is bonded to C to form carbides andthereby contributes to enhancing the strength of the steel sheet. Toensure the desired strength (tensile strength: 980 MPa or more) of thehot-rolled steel sheet, Ti content is preferably 0.14% or more. On theother hand, if Ti content in steel exceeds 0.20% in producing ahot-rolled steel sheet, coarse Ti carbides may not be dissolved byheating the steel material (slab) prior to hot rolling, which results incoarse Ti carbides remaining in the finally obtained (coiled) hot-rolledsteel sheet. Such coarse Ti carbides left in the steel sheet reduce thestrength of the hot-rolled steel sheet, and even more, interfacesbetween the matrix and the coarse Ti carbides serve as the origins ofvoids when the steel sheet is being subjected to bending working, whichresults in a deterioration in the bending workability of the steelsheet. Accordingly, the Ti content is to be 0.14% or more and 0.20% orless, preferably 0.15% or more and 0.19% or less.

0.07%≦V≦0.14%

Vanadium (V) is an element that is bonded to C to form carbides andthereby contributes to enhancing the strength of the steel sheet, as isthe case with Ti. V is bonded to Ti to form fine composite carbides, andthus is effective for enhancing the strength of the steel sheet. Toensure the desired strength (tensile strength: 980 MPa or more) of thehot-rolled steel sheet, V content is preferably 0.07% or more. On theother hand, if V content is larger than the Ti content, it is difficultto allow V to precipitate, in which case more V would remain in thesteel sheet in solute state. Since V in solute state leads to adeterioration in the bending workability of the steel sheet, the Vcontent is preferably equal to or smaller than the Ti content, i.e.,0.14% or less. Accordingly, the V content is to be 0.07% or more and0.14% or less, preferably 0.08% or more and 0.13% or less.

0.01%≦Nb≦0.05%

In addition to the aforementioned basic components, the composition ofthe steel sheet of the present invention may further contain Nb: 0.01%or more and 0.05% or less.

Niobium (Nb) is an element that acts to inhibit recrystallization ofaustenite grains before transforming from austenite to ferrite andthereby provide non-recrystallized structure in the hot rolling step inproducing a hot-rolled steel sheet having substantially ferritesingle-phase structure. As the non-recrystallized structure is moreprone to storage of strain energy caused by hot rolling, there are morenucleation sites for ferrite phase. Thus, addition of Nb may increasethe number of nucleation sites for ferrite phase in the hot rollingstep, which enables refinement of grains of the ferrite phase. To obtainthis effect, Nb content is preferably 0.01% or more. However, ifexcessive strain energy is applied to the steel in the hot rolling step,the temperature at which austenite to ferrite transformation occurs israised, in which case fine carbides may not be obtained. In view ofthis, Nb content is preferably 0.05% or less, more preferably 0.02% ormore and 0.04% or less.

Mo≦0.05%, W≦0.05%, Zr≦0.05%, Hf≦0.05%

In addition to the aforementioned basic components, the composition ofthe steel sheet of the present invention may further contain at leastone of molybdenum (Mo), tungsten (W), zirconium (Zr) and hafnium (Hf),in which case the content of each element is preferably controlled sothat Mo: 0.05% or less, W: 0.05% or less, Zr: 0.05% or less, and Hf:0.05% or less.

Mo, W, Zr and Hf are elements that form carbides contributing toenhancing the strength of the steel sheet; however, they remain in thesteel sheet in solute state in a large amount. These solute elementsdeteriorate the workability of the matrix and adversely affect thebending workability of the steel sheet. Mo, W, Zr and Hf precipitate ata low rate relative to their contents and remain in the steel sheet assolute elements in a large amount. Accordingly, it is desirable toreduce the contents of these elements as much as possible; however thecontent of each of these elements is acceptable up to 0.05%, and thusthe upper limit of each element is to be 0.05%. Preferably, the contentof each element is 0.03% or less. Also, the contents of Mo, W, Zr and Hfmay be zero.

Other Possible Elements

In addition to the aforementioned basic components, the composition ofthe steel sheet of the present invention may further contain at leastone of O (oxygen), Se, Te, Po, As, Bi, Ge, Pb, Ga, In, T, Zn, Cd, Hg,Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re, Ta, Be, Sr, REM, B, Ni, Cr,Sb, Cu, Sn, Mg, and Ca, in a total amount of 0.2% or less. From theviewpoint of the bending workability of the steel sheet, acceptablecontents of these elements are up to 0.2% in total, preferably not morethan 0.09% in total. The balance, or components other than thosedescribed above, of the composition of the steel sheet is Fe andincidental impurities.

In addition, a plating layer may also be formed on a surface of thehot-rolled steel sheet of the present invention. Formation of such aplating layer on the surfaces improves the corrosion resistance propertyof the hot-rolled steel sheet, and thereby makes the steel sheetapplicable to components such as automobile components that are used ina severe corrosion environment.

The present invention is not limited to a particular type of platinglayer, and so both an electroplated layer and an electroless-platedlayer are applicable as the plating layer. Also, in the presentinvention, there is no particular limitation on the alloy components ofthe plating layer, and preferred examples of the alloy componentsinclude a hot-dip galvanized layer, a hot-dip galvannealed layer, and soon. Of course, however, the present invention is not limited to thedisclosed components, and so any conventionally known components areapplicable.

(Method of Producing a High-Strength Hot-Rolled Steel Sheet)

Next, an exemplary method for producing the hot-rolled steel sheet ofthe present invention will be described.

The method of an embodiment of the present invention comprises: heatinga steel material (slab) having the above-described composition;subjecting the steel material to hot rolling including rough rolling andfinish rolling; and after completion of the finish rolling, cooling andcoiling the steel material to gain a hot-rolled steel sheet.

In the method of the present invention, the steel material is preferablyheated at a temperature of 1100° C. or higher and 1350° C. or lower, thefinish rolling is operated at a finish rolling temperature of 850° C. orhigher, the cooling is initiated within 3 seconds after completion ofthe finish rolling, the cooling is operated at an average cooling rateof 20° C./s or higher, and the coiling is operated at a coilingtemperature of 550° C. or higher and 700° C. or lower.

The present invention is not limited to a particular steelmaking method,and so any known steelmaking method may be adopted, such as usingconverter, electric furnace, and so on. In addition, secondaryrefinement may also be operated in a vacuum degassing furnace. Then,while continuous casting is preferably used to cast a slab (steelmaterial) in terms of productivity and quality, any known casting methodmay also be used to cast a slab, such as ingot casting-blooming or thinslab continuous casting.

Heating Temperature of Steel Material: 1100° C. to 1350° C.

The steel material (steel slab) thus obtained is subjected to roughrolling and finish rolling. And, in the present invention, the steelmaterial is preferably heated prior to the rough rolling so as toestablish substantially uniform austenite phase and dissolve coarsecarbides. If the steel material is heated at a temperature below 1100°C., coarse carbides are not dissolved, and therefore, less carbides aresubjected to fine particle distribution at the cooling and coiling stepafter completion of the hot rolling. This results in a significantdeterioration in the strength of the finally obtained hot-rolled steelsheet. Alternatively, if the heating temperature is above 1350° C.,scale defects occur, degrading the surface appearance quality of thesteel sheet.

For this reason, the steel material is to be heated at a temperature of1100° C. or higher and 1350° C. or lower, preferably 1150° C. or higherand 1320° C. or lower. However, when the steel material is subjected tohot rolling, and if the steel material after casting is at a temperaturerange of 1100° C. or higher to 1350° C. or lower, or if the carbides inthe steel material have been dissolved, then the steel material may besubjected to hot direct rolling without being heated. The presentinvention is not limited to a particular rough rolling condition.

Finish Rolling Temperature: 850° C. or Higher

If the finish rolling temperature is below 850° C., ferritetransformation begins during the finish rolling, which results inmicrostructures with extended ferrite grains, and furthermore, mixedgrain size microstructures with partially grown ferrite grains. Thissignificantly deteriorates the bending workability of the hot-rolledsteel sheet. Accordingly, the finish rolling temperature is to be 850°C. or higher, preferably 870° C. or higher. While the upper limit of thefinish rolling temperature is not particularly specified herein, thefinish rolling temperature is determined by the slab reheatingtemperature, sheet passage rate and steel sheet thickness. Thus, theupper limit of the finish rolling temperature is substantially 990° C.or lower.

Time to Initiate Forced Cooling after Completion of the Finish Rolling:Within 3 Seconds

In the steel sheet under high temperature condition immediately afterthe finish rolling, carbides are caused by strain-induced precipitationdue to large strain energy stored in the austenite phase. Since suchcarbides are susceptible to coarsening as they precipitate at hightemperature, the occurrence of strain-induced precipitation makes itdifficult to obtain fine precipitates. Accordingly, the presentinvention preferably includes forced cooling that is initiated promptlyafter completion of the hot rolling for the purpose of suppressingstrain-induced precipitation, and therefore, cooling is initiated within3 seconds at the latest, preferably within 2 seconds, after completionof the finish rolling in the present invention.

Average Cooling Rate: 20° C./s or Higher

As described above, the longer the steel sheet stayed at hightemperature after completion of the finish rolling, the more thecarbides prone to coarsening caused by strain-induced precipitation. Inaddition, while the present invention optionally suppresses austenite toferrite transformation by means of a predetermined amount of Mncontained in the steel sheet, ferrite transformation would begin at hightemperature if the cooling rate is low, in which case carbides are moresusceptible to coarsening. Thus, rapid cooling is required after thefinish rolling, and the steel sheet is preferably cooled at an averagecooling rate of 20° C./s or higher to avoid the above-describedproblems. The average cooling rate is preferably 40° C./s or higher.However, if the cooling rate is excessively increased after completionof the finish rolling, there is a concern that it becomes more difficultto control coiling temperature and to obtain stable strength of thehot-rolled steel sheet. Therefore, the average cooling rate ispreferably not higher than 150° C./s.

Coiling Temperature: 550° C. to 700° C.

If the coiling temperature is below 550° C., it is not possible toobtain a sufficient amount of carbides, which results in a deteriorationin the strength of the steel sheet. On the other hand, if the coilingtemperature exceeds 700° C., the precipitated carbides coarsen andtherefore the strength of the steel sheet is reduced. Accordingly, thecoiling temperature is to be 550° C. or higher and 700° C. or lower,preferably 580° C. or higher and 680° C. or lower.

Additionally, the hot-rolled steel sheet having been subjected to hotrolling and coiling has such properties that will not change whether ina state where scales are attached to the surfaces or in a state wherescales have been removed by pickling. In both of these states, thehot-rolled steel sheet exhibits excellent properties as described above.In the present invention, the hot-rolled steel sheet after coiling mayalso be subjected to plating treatment so that a plating layer isprovided on a surface of the hot-rolled steel sheet.

The hot-rolled steel sheet shows a small variability of materialproperties even when subjected to heating treatment up to 740° C. for ashort period of time. Thus, for the purpose of imparting a corrosionresistance property to the hot-rolled steel sheet of the presentinvention, the steel sheet may be subjected to plating treatment toprovide a plating layer on a surface thereof. Since the hot-rolled steelsheet of the present invention can be produced when heated at atemperature of 740° C. or lower during plating treatment, the hot-rolledsteel sheet may be subjected to plating treatment without loss of theabove-described effects of the present invention. The present inventionis not limited to a particular type of plating layer, and so both anelectroplated layer and an electroless-plated layer are applicable asthe plating layer. Also, in the present invention, there is noparticular limitation on the alloy components of the plating layer, andpreferred examples of the alloy components include a hot-dip galvanizedlayer, a hot-dip galvannealed layer, and so on. Of course, however, thepresent invention is not limited to the disclosed components, and so anyconventionally known components are applicable.

Moreover, the present invention is not limited to a particular platingtreatment method, and so any conventionally known methods areapplicable. Exemplary methods include passing a steel sheet through acontinuous galvanizing/galvannealing line with an annealing temperatureof 740° C. or lower, followed by immersing the steel sheet in a moltenbath and then lifting it from the molten bath. After the platingtreatment, the steel sheet may also be subjected to alloying treatmentby heating the surfaces of the steel sheet in a furnace, such as a gasfurnace.

As described above, the present invention may provide such a hot-rolledsteel sheet, by optimizing the composition and producing conditionsthereof, that has microstructures such that an area ratio of ferritephase is 95% or more, an average grain size of the ferrite phase is 8 μMor less, and carbides in grains of the ferrite phase have an averageparticle size of less than 10 nm. In addition, the present inventionincludes enhancing the strength of the steel sheet, while reducingsolute elements and coarse inclusions present in the steel sheet for thepurpose of improving the bending workability of the steel sheet. Assuch, the high-strength hot-rolled steel sheet according to the presentinvention may have excellent bending workability.

Moreover, the present invention specifies the producing conditions ofthe hot-rolled steel sheet, while optimizing the contents ofcarbide-forming elements (Ti and V, and furthermore, Nb, W, Mo, Hf andZr) contained in the steel sheet. This allows the above-describedcarbides having an average particle size of less than 10 nm to beprecipitated in the ferrite grains sufficiently, and the tensilestrength of the hot-rolled steel sheet to be increased to 980 MPa ormore, while maintaining excellent bending workability of the steelsheet. It should be noted that the present invention is preferablyapplied to a hot-rolled steel sheet having a tensile strength of 1100MPa or less, more preferably 1052 MPa or less.

EXAMPLES

Steel materials (steel slabs) of 250 mm thick having the compositionsshown in Table 1 were subjected to hot rolling under the hot rollingconditions shown in Table 2 to gain hot-rolled steel sheets having asheet thickness of 1.4 mm to 3.2 mm, respectively. The cooling rateshown in Table 2 indicates the average cooling rate from the finishrolling temperature to the coiling temperature.

In addition, some of the resulting hot-rolled steel sheets were passedthrough a hot-dip galvanizing line with an annealing temperature of 720°C., and then immersed in a molten bath at 460° C. (plating composition:Zn—0.13 mass % Al), whereby hot-dip galvanized materials (GI materials)were obtained. Further, subsequent to the sheet passage through thehot-dip galvanizing line and the following immersion in the molten bath,some of the hot-dip galvanized materials (GI materials) were subjectedto alloying treatment at 520° C., whereby galvannealed materials (GAmaterials) were obtained. For both GI and GA materials, the coatingweight was 45 g/m² to 55 g/m² per surface.

Besides, it was separately ascertained that austenite to ferritetransformation had not occurred during the cooling step until coiling,except for Steel Sheet Nos. 3 to 5 and 12 to 18.

TABLE 1 Chemical Composition (mass %) Steel C Si Mn P S Al N Ti V Nb Mo,W, Zr, Hf Others Remarks A 0.081 0.01 1.05 0.01 0.0056 0.041 0.00380.158 0.10 — — — Conforming Steel B 0.079 0.02 0.85 0.02 0.0051 0.0410.0029 0.186 0.12 — — — Conforming Steel C 0.089 0.01 1.18 0.02 0.00480.041 0.0039 0.167 0.12 0.02 — O: 0.0009, Bi: 0.0001, Conforming SteelGe: 0.0009, Pb: 0.0001, Cd: 0.0001, Pt: 0.0001, Co: 0.002, Re: 0.0001 D0.085 0.02 1.02 0.01 0.0053 0.045 0.0026 0.148 0.13 — Zr: 0.02 Ni:0.021, Conforming Steel Cr: 0.026, Cu: 0.09, As: 0.0008, REM: 0.002, B:0.0002, Hg: 0.0001, Ag: 0.0001, Rh: 0.0001, Au: 0.0001, Pd: 0.0001 E0.085 0.02 1.05 0.01 0.0057 0.038 0.0048 0.164 0.11 — — Zn: 0.0008,Conforming Steel Ir: 0.0002, Ru: 0.0002, Tn: 0.001, Sb: 0.001, Mg:0.002, Ti: 0.0001, Os: 0.0001, Ga: 0.0002 F 0.081 0.01 1.07 0.02 0.00510.041 0.0039 0.168 0.12 — Mo: 0.02, In: 0.0001, Ca: 0.002, ConformingSteel Hf: 0.01 Po: 0.0002, Sn: 0.01 G 0.081 0.01 1.10 0.02 0.0012 0.0390.0028 0.169 0.11 — W: 0.02 Se: 0.0001, Conforming Steel Te: 0.0001, Be:0.0002, Sr: 0.0002, Tc: 0.0001 H 0.052 0.02 1.11 0.01 0.0051 0.0410.0033 0.158 0.10 — — — Comparative Steel I 0.112 0.01 1.03 0.01 0.00110.040 0.0029 0.166 0.11 — — — Comparative Steel J 0.082 0.50 1.12 0.020.0015 0.046 0.0034 0.169 0.10 — — — Comparative Steel K 0.085 0.01 0.410.02 0.0035 0.041 0.0036 0.166 0.11 — — — Comparative Steel L 0.081 0.031.52 0.01 0.0051 0.040 0.0023 0.161 0.12 — — — Comparative Steel M 0.0820.01 1.15 0.01 0.0032 0.041 0.0035 0.130 0.10 — — — Comparative Steel N0.085 0.03 1.09 0.01 0.0051 0.040 0.0023 0.161 0.03 — — — ComparativeSteel * Values underlined if out of the scope of the present invention.

TABLE 2 Hot Rolling Step Steel Seet Slab Heating Finish Rolling Time toInitiate Average Cooling Coiling Temp. No. Steel Temp. (° C.) Temp. (°C.) Cooling *1 (s) Rate (° C./s) (° C.) Remarks  1 A 1250 920 1.2 50 580Inventive Example  2 1260 910 1.1 55 650 Inventive Example  3 1250 9201.2  5 620 Comparative Example  4 1250 910 1.0 55 520 ComparativeExample  5 1240 910 1.1 60 750 Comparative Example  6 B 1250 920 0.8 55620 Inventive Example  7 C 1280 940 1.1 60 610 Inventive Example  8 D1260 930 1.0 60 620 Inventive Example  9 E 1270 960 1.5 65 610 InventiveExample 10 F 1260 930 1.1 60 630 Inventive Example 11 G 1250 920 1.8 80610 Inventive Example 12 H 1260 920 1.1 60 630 Comparative Example 13 I1250 910 1.3 60 620 Comparative Example 14 J 1270 930 1.0 65 600Comparative Example 15 K 1250 910 0.9 55 620 Comparative Example 16 L1260 900 1.2 60 610 Comparative Example 17 M 1270 930 1.0 65 630Comparative Example 18 N 1250 920 1.3 60 590 Comparative Example *Values underlined if out of the scope of the present invention. *1: Timeto initiate cooling after completion of the finish rolling (in seconds).

Test specimens were taken from the hot-rolled steel sheets thus obtained(hot-rolled steel sheets, GI materials and GA materials) and subjectedto the microstructure observation, tensile test and bend test todetermine the following: area ratio of ferrite phase; types and arearatios of phases other than ferrite phase; average grain size of ferritephase; average particle size of carbides; yield strength; tensilestrength; elongation; and limit bending radius. The test method was asfollows.

(i) Microstructure Observation

The area ratio of ferrite phase was evaluated in the followingprocedure. At the central portion of the sheet thickness in across-section parallel to the rolling direction, 10 fields ofmicrostructures on which corrosion emerged with 5% nital werephotographed under a scanning optical microscope at 400× magnification.Ferrite phase is such a phase with no corrosion traces or no cementiteobserved in the grains thereof. In addition, assuming ferrite includespolygonal ferrite, bainitic ferrite, acicular ferrite and granularferrite, the following parameters were derived: area ratio of ferritephase; average grain size of ferrite phase; and average particle size ofcarbides in grains of the ferrite phase.

The area ratio of ferrite phase was determined by image analysismeasuring an area ratio of ferrite phase to the observed field, whileseparating the ferrite phase from other phases such as bainite ormartensite phases. In this case, grain boundaries appeared in linearform were construed as part of ferrite phase. The obtained results onthe area ratio of ferrite phase are shown in Table 3.

The average grain size of ferrite phase was determined by using anintersection method under ASTM E 112-10, where three horizontal linesand three vertical lines were respectively drawn in representative threeimages, among those taken at 400× magnification as described earlier, tocalculate an average among the three images, which was considered as thefinal average grain size. The obtained results on the average grain sizeare shown in Table 3.

The average particle size of carbides in grains of the ferrite phase wasdetermined by using a microfilm method to fabricate samples from thecentral portion of the sheet thickness of each hot-rolled steel sheetobtained, which samples were then observed under a transmissionelectronic microscope (at 135,000× magnification) to calculate anaverage of the precipitate particle size measurements at 100 points ormore. In calculating the precipitate particle size, coarse cementite andnitrides having a grain size of 1.0 μm or more were excluded from thecalculation. The obtained results on the average particle size ofcarbides are shown in Table 3.

(ii) Tensile Test

JIS No. 5 tensile test specimens were fabricated from the resultinghot-rolled steel sheets in a direction perpendicular to the rollingdirection, and then subjected to tensile tests five times pursuant toJIS Z 2241 (2011) standard to determine the average values of yieldstrength (YS), tensile strength (TS) and total elongation (El). Besides,the tensile tests were conducted with crosshead speed of 10 mm/min.

(iii) Bend Test (for Evaluating Bending Workability)

Strip test specimens (100 W mm×35 L mm) were taken from the resultinghot-rolled steel sheets by shearing work so that their longitudinaldirection is vertical to the rolling direction. In this case, a shearedsurface and a fractured surface were directed in the same direction atan edge face of each test specimen.

Each test specimen thus obtained was subjected to bend tests three timesusing the V-block bend test pursuant to JIS Z 2248, and after the tests,the external appearance of the curved portions of the samples werevisually observed, where those samples were considered as having passedthe tests if no defects, such as cracks or scars, were observed on thecurved portion thereof. The bend tests were carried out by usingindenters having different inside radii, where, as shown in thefollowing formula, the minimum inside radius R (mm) of each successfulindenter (an indenter with which samples have passed the tests) wasdivided by the sheet thickness t (mm) of the hot-rolled steel sheet, andthe result (R/t) was determined as the limit bending radius:

(limit bending radius)=(minimum inside radius of successful indenterR)/(sheet thickness of steel sheet t).

A smaller limit bending radius means a better result. Circle representsa good result where the limit bending radius is not more than 2.0, whilecross indicates a poor result where the limit bending radius is morethan 2.0.

The obtained results are shown in Table 3.

TABLE 3 Microstructure of Hot Rolled Steel Sheet Mechanical Propertiesof Hot Rolled Steel Sheet Steel Steel Thickness of Area Ratio of FerriteGrain Particle Size of Yield Strength Tensile Strength Elongation SheetHot Rolled Steel Sheet Ferrite Phase Size *3 Carbides *4 YS TS E1 No.(mm) Plating 2* (%) (μm) (nm) (MPa) (MPa) (%)  1 2.0 — 100 2.8 2 924 995 19  2 2.3 — 100 3.9 3 913  981 20  3 2.0 — 100 3.5 10  730  785 22 4 2.0 — 89 (balance:bainite) 2.7 2 841  927 15  5 2.0 — 100 9   11  628 675 23  6 1.6 — 100 3.2 2 975 1052 18 1.6 GI 100 3.3 2 974 1049 18  71.4 — 100 3.1 2 952 1028 18 1.4 GA 100 3.2 3 953 1025 18  8 1.8 — 1003.3 2 942 1013 19 1.8 GA 100 3.2 2 935 1010 19  9 3.2 — 100 4.5 3 9611030 20 10 2.3 — 100 3.2 3 952 1014 19 11 1.8 — 100 4.1 2 961 1029 19 121.6 — 100 4.1 2 834  912 20 13 1.4 — 92 (balance:pearlite) 3.8 3 921 995 18 14 1.6 — 100 3.5 2 950 1016 18 15 1.2 — 100 6.1 10  851  924 2016 1.6 — 100 3.5 2 949 1020 18 17 1.8 — 100 3.5 3 888  955 20 18 2.0 —100 3.8 3 854  918 21 Steel Bending Workability Sheet Limit BendingEvaluation No. Radius *5 Remarks  1 0.5 ◯ Inventive Example  2 0.4 ◯Inventive Example  3  <0.25   ◯ Comparative Example  4 4.0 X ComparativeExample  5  <0.25   ◯ Comparative Example  6 0.6 ◯ Inventive Example 0.6◯ Inventive Example  7 0.7 ◯ Inventive Example <0.4   ◯ InventiveExample  8 0.6 ◯ Inventive Example 0.6 ◯ Inventive Example  9 0.9 ◯Inventive Example 10 1.3 ◯ Inventive Example 11 0.6 ◯ Inventive Example12 0.6 ◯ Comparative Example 13 >5.7   X Comparative Example 14 >5.0   XComparative Example 15 1.7 ◯ Comparative Example 16 >5.0   X ComparativeExample 17 0.6 ◯ Comparative Example 18 1.0 ◯ Comparative Example *Values underlined if out of the scope of the present invention. *2“—”indicates a hot rolled steel sheet without plating. “GI” indicates a hotrolled steel sheet with a hot-dip galvanized layer. “GA” indicates a hotrolled steel sheet with a hot-dip galvannealed layer. *3 Average crystalgrain size of ferrite phase. *4 Average particle size of carbides inferrite grains. *5 Cirlce (“◯”) indicates where the limit bending radiusis not more than 2.0. Cross (X) indicates where the limit bending radiusis more than 2.0.

It was found that all of the inventive examples provide hot-rolled steelsheets balancing strength and workability, having a high tensilestrength TS of 980 MPa or more and excellent bending workability. Incontrast, it was revealed that the comparative examples out of the scopeof the present invention fail to demonstrate a predetermined high levelof strength, or otherwise fail to offer good bending workability.

According to the present invention, it is possible to obtain ahigh-strength hot-rolled steel sheet having a tensile strength of 980MPa or more and excellent bending workability that is suitablyapplicable to automobile structural members or the like, ensuring bothreduction in the weight of automobile members and formation ofautomobile members.

1-14. (canceled)
 15. A high-strength hot-rolled steel sheet comprising achemical composition containing, in mass %, C: 0.06% or more and 0.1% orless, Si: 0.09% or less, Mn: 0.7% or more and 1.3% or less, P: 0.03% orless, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less, Ti: 0.14% ormore and 0.20% or less, V: 0.07% or more and 0.14% or less, and thebalance being Fe and incidental impurities, wherein the steel sheet hasmicrostructures such that an area ratio of ferrite phase is 95% or more,an average grain size of the ferrite phase is 8 μm or less, and carbidesin grains of the ferrite phase have an average particle size of lessthan 10 nm, and wherein the steel sheet has a tensile strength of 980MPa or more.
 16. The high-strength hot-rolled steel sheet according toclaim 15, wherein the chemical composition further contains at least onegroup selected from (A) to (C), wherein (A) in mass %, Nb: 0.01% or moreand 0.05% or less (B) in mass %, at least one element selected from Mo:0.05% or less, W: 0.05% or less, Zr: 0.05% or less, and Hf: 0.05% orless, (C) in mass %, at least one of O (oxygen), Se, Te, Po, As, Bi, Ge,Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re,Ta, Be, Sr, REM, B, Ni, Cr, Sb, Cu, Sn, Mg, and Ca, in a total amount of0.2% or less.
 17. The high-strength hot-rolled steel sheet according toclaim 15, further comprising a plating layer on a surface of the steelsheet.
 18. The high-strength hot-rolled steel sheet according to claim16, further comprising a plating layer on a surface of the steel sheet.19. The high-strength hot-rolled steel sheet according to claim 17,wherein the plating layer is a galvanized layer.
 20. The high-strengthhot-rolled steel sheet according to claim 17, wherein the plating layeris a galvannealed layer.
 21. The high-strength hot-rolled steel sheetaccording to claim 18, wherein the plating layer is a galvanized layer.22. The high-strength hot-rolled steel sheet according to claim 18,wherein the plating layer is a galvannealed layer.
 23. A method ofproducing a high-strength hot-rolled steel sheet, comprising: heating asteel material; subjecting the steel material to hot rolling includingrough rolling and finish rolling; and after completion of the finishrolling, cooling and coiling thus rolled steel material to gain ahot-rolled steel sheet, wherein the steel material has a chemicalcomposition containing, in mass %, C: 0.06% or more and 0.1% or less,Si: 0.09% or less, Mn: 0.7% or more and 1.3% or less, P: 0.03% or less,S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less, Ti: 0.14% or moreand 0.20% or less, V: 0.07% or more and 0.14% or less, and the balancebeing Fe and incidental impurities, and wherein the steel material isheated at a temperature of 1100° C. or higher and 1350° C. or lower, thefinish rolling is operated at a finish rolling temperature of 850° C. orhigher, the cooling is initiated within 3 seconds after completion ofthe finish rolling, the cooling is operated at an average cooling rateof 20° C./s or higher, and the coiling is operated at a coilingtemperature of 550° C. or higher and 700° C. or lower.
 24. The method ofproducing a high-strength hot-rolled steel sheet according to claim 23,wherein the chemical composition further contains at least one groupselected from (A) to (C), wherein (A) in mass %, Nb: 0.01% or more and0.05% or less (B) in mass %, at least one element selected from Mo:0.05% or less, W: 0.05% or less, Zr: 0.05% or less, and Hf: 0.05% orless, (C) in mass %, at least one of 0 (oxygen), Se, Te, Po, As, Bi, Ge,Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re,Ta, Be, Sr, REM, B, Ni, Cr, Sb, Cu, Sn, Mg, and Ca, in a total amount of0.2% or less.
 25. The method of producing a high-strength hot-rolledsteel sheet according to claim 23, comprising forming a plating layer ona surface of the hot-rolled steel sheet.
 26. The method of producing ahigh-strength hot-rolled steel sheet according to claim 24, comprisingforming a plating layer on a surface of the hot-rolled steel sheet. 27.The method of producing a high-strength hot-rolled steel sheet accordingto claim 25, wherein the plating layer is a galvanized layer.
 28. Themethod of producing a high-strength hot-rolled steel sheet according toclaim 25, wherein the plating layer is a galvannealed layer.
 29. Themethod of producing a high-strength hot-rolled steel sheet according toclaim 26, wherein the plating layer is a galvanized layer.
 30. Themethod of producing a high-strength hot-rolled steel sheet according toclaim 26, wherein the plating layer is a galvannealed layer.